1. Field of the Invention
This invention relates to apparatus for generating biphase waveforms and particularly to means of the type described employing shift registers. More particularly, this invention relates to a biphase waveform generator using a serially arranged shift register of length 2N where N is the number of bits to be transmitted.
2. Description of the Prior Art
Many of the digital communications systems now in use employ biphase waveforms. The advantage of this type of waveform is its self clocking characteristic. When transmitted as a bi-polar waveform the benefits of differential transmission lines add to its desirability as a form of communication. One of the more widely used biphase waveforms is that known as the Manchester code. The present invention deals with apparatus for generating such a code.
The classical approach to the aforenoted code generation rests on the fact that an "exclusive-or" relationship exists between the biphase waveform and the NRZ binary data i.e., data that does not return to zero. A shift clock is used to create the waveform. Typically, an eight bit shift register is loaded with the binary input information to be transmitted. When the shift clock is active, data is shifted out of the register. The clock and register outputs are "exclusive-or-ed", and the result is enveloped with a "transmit enable" signal which produces the desired biphase waveform or Manchester word. The resulting waveform may be the input to a differential output stage for driving a transformer coupled transmission line. Inverters may be provided before the "exclusive or" gate operation to match the propagation delay of the shift register for providing coincidence of the waveforms. At higher clocking rates, this coincidence or alignment as well as the differences between rise and fall times becomes increasingly more critical.